Previous indications that platelet monoamine oxidase (MAO) activity differences may reflect vulnerability to psychopathology have been supported by further studies in normal populations. A follow-up investigation of individuals identified on the basis of differences in MAO activity as well as cortical evoked response differences revealed a higher incidence of new affective disorders in these same subgroups. Platelet MAO activity differences were also found to be related to other types of vulnerability, including the risks of developing behavioral side effects from disulfiram and MAO-inhibiting antidepressants. In laboratory studies, large primate-rodent species differences in the proportion of the MAO subtypes MAO-A and MAO-B were demonstrated. Brain tissue from three primate species (human, rhesus and vervet) all possessed substantially higher proportions of MAO-B than MAO-A activity across different brain regions, while rats, mice and hamsters all had higher MAO-A activity. These differences raise questions about the applicability of rodent models in studies of some aspects of brain amine metabolism and amine-affective drugs.